It fermented

What is happening

Yeast tolerant of high sugar is working on the syrup and producing gas and alcohol. It needs available water, so like mold it is a water activity failure, which is a density failure stated differently. A properly finished syrup does not do this.

Where formulated syrups are vulnerable

This is the common failure in simple and bar syrups, because there is no identity standard pushing the concentration up and the maker chose the number. A thin syrup intended to keep on a shelf will ferment, and it will do it faster warm. Many of these products are properly refrigerated products and the label has to say so.

Fruit and infusions dilute

Juice, fruit and fresh botanicals bring water, and they also bring the yeast. A base syrup at a safe concentration can be pushed below it by the flavoring added afterwards, which is why the measurement belongs on the finished product and not on the base.

Pressure is a handling issue too

A fermenting bottle builds pressure, and a closure that holds is a closure under load. Treat an affected batch as something to open carefully and dispose of rather than to taste, and recall the batch code rather than the one bottle.

Warm storage is the accelerant

A marginal syrup that behaves in a cool storeroom will ferment in a vehicle in summer or on a shelf near a window. That is worth knowing because it means a product can pass every check you run and fail in distribution, and the fix is density rather than asking a retailer to store it carefully.

References